1. Field of the Invention
The present invention generally relates to the design and testing of circuits, such as integrated circuits and more particularly relates to methods and devices for accurately testing circuits having open drain or open collector outputs during the design and manufacture of these circuits.
2. State of the Art
Testings devices, or testers, are currently used for establishing a test set up to check the functionality of circuits, such as integrated circuits. Integrated circuits, including application specific integrated circuits (ASICs), normally have one or more functional blocks or megafunctions, each of which is composed of any number of logic gates. Further, a circuit designer will typically include integrated circuit output pads for generating open collector or open drain outputs. Output pads are physical outputs of an integrated circuit for connecting with other external devices.
For example, an open drain output pad can provide a logic level high output state and a high impedance output state. When in its high impedance state, the logic high output state is typically pulled down to a logic level low by an external circuit which, for example, includes a pull-down resistor. This output state is controlled in response to internal conditions of a pad driver circuit in conjunction with the condition of the external circuit connected to the pad output. Subsequently, internal conditions of the circuit can be modified to once again drive the output to its logic level high state as will be described below.
Similarly, an open collector output pad can provide a logic level low output and a high impedance state. During the high impedance state, the pad output is pulled up to a logic level high. Afterwards, internal conditions of the circuit can be modified to again drive the output to a logic level low.
The internal inputs to an integrated circuit output pad typically include an output enable control signal and an input drive signal. The output enable control signal determines the output condition of the pad. For example, when an active low output enable control signal is high (i.e., inactive condition), the output pad is placed into its high impedance state. On the contrary, when the active low output enable control signal is low (i.e., active condition), the output pad is actively driven to the logic state dictated by the input drive signal.
Thus, open collector and open drain output pads are valuable to the circuit designer because their output can be driven to a known logic state and, afterwards transitioned to another logic state. These output pads are often included on semiconductor chips for initiating an interrupt condition. The output pads of different devices can be connected via a wired "AND" so that plural interrupts can be generated over a single interrupt line.
For example, to initiate an interrupt with an open collector, the open collector output is driven from a pulled-up high impedance state to a low logic level. After the interrupt has been serviced, the enable control signal of the open collector is deactivated such that external circuitry can again pull-up the open collector pad output and return it to a logic level high. A pull-up device associated with the external circuitry causes the open collector pad output to rise, or "float" to the inactive logic level high condition at a rate proportional to an RC time constant associated with the pull-up device.
More particularly, an external resistor (i.e., either pull-up or pull-down) is typically associated with a given output pad. The same external pull-up or pull-down circuitry can be associated with plural output pads combined to form a net of potential interrupting devices. When a pad is placed into its high impedance state, the resistance value and attendant capacitance of circuits connected to that pad constitute an RC time constant. The high impedance state (or tri-state) is thus characterized by resistance and capacitance which affects a transition of the pad output to the logic level associated with the high impedance state (i.e., transition to the high impedance state experiences a time delay due to the RC time constant).
To ensure that a circuit having open collector and/or open drain output pads will operate properly, it is desirable to use a test set up which can simulate circuits to be connected to the output pads. Functionality testing of a circuit is necessary to identify flaws due, for example, to incorrect logic designs in the circuit, timing problems and manufacturing defects. However, the testing of circuits which include open collector and open drain output pads poses several difficulties which often result in inaccurate testing.
During integrated circuit testing, a typical testing device will simulate the output impedance associated with open collector or open drain output pads. However, timing considerations related to the RC constant of the test environment (e.g., the testing device) will affect both the performance of the output pad as well as the proper point in time to test for a valid output during an inactive output enable control signal. For example, the pad output can be instantaneously driven to an active state in response to an output enable control signal, the float time required for the output to achieve its inactive, high impedance state is dependent upon the RC conditions of the test environment.
Because the RC time constant will vary from test set up to test set up, a test time at which the output pad is examined to determine whether its output has properly transitioned during an inactive output state will be different for different testers. Further, the numerous potential output drive/output capacitive loading combinations which exist render the use of software-based testing techniques infeasible.
Accordingly, it would be desirable to provide accurate techniques and devices for testing circuits designed with open collector and/or open drain outputs.